Recently, there have been developed techniques for treating waste resin materials such as polystyrene foam, which have an apparent volume larger than the actual solid (resin component) volume (see, for example, Japanese Patent Application Laid-Open (kokai) Nos. 2-1748, 5-59212, 7-113089, 9-40802, and 9-157435, 2001-181439). According to these techniques, waste resin material is brought into contact with a solvent to form a gel, whereby the volume of the waste resin material is reduced to a volume almost equivalent to the actual volume. Then, the resin is separated from the solvent and reused as a fuel or a resin raw material, while the solvent is reused as a solvent for the volume reduction treatment.
In one procedure proposed in the above, relatively new technical field, resin (solid) and solvent (liquid) are separated from a gel product through evaporation of the solvent to thereby yield a solid, and the vapor is condensed to recover the solvent. Generally, the treatment in which waste resin material is brought into contact with a solvent to form a gel, whereby the volume of the waste resin material is reduced to a volume almost equivalent to the actual volume, is referred to as volume-reduction gelling or simply as volume reduction. The resin that has been gelled to gelatinous solid or gel is called volume-reduced gel-state polystyrene resin.
The aforementioned volume-reduced gel-state polystyrene resin produced from collected waste resin material contains foreign matter such as adhesive tape or labels. Therefore, such foreign matter must be removed from the volume-reduced gel-state polystyrene resin through filtration before carrying out separating/recovering resin and solvent. One disclosed filtration apparatus for volume-reduced gel-state polystyrene resin is an apparatus in which volume-reduced gel-state polystyrene resin is caused to pass through filters (e.g., mesh filters) which are disposed on the top and bottom sides of a high-temperature bath. Another filtration apparatus is an impurity removal apparatus in which volume-reduced gel-state polystyrene resin is fed to a hollow cylindrical filter and filtered through centrifugation by rotating the cylindrical filter (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2001-164035).
When such an impurity removal apparatus is employed, foreign matter contained in the volume-reduced gel-state polystyrene resin can be removed at relatively high efficiency. However, the thus-removed foreign matter is gradually accumulated in the apparatus, requiring frequent maintenance operations. Thus, such an apparatus attains low treatment efficiency with high treatment cost, which is problematic.